Ambulatory infusion device with sensor testing unit

ABSTRACT

An ambulatory infusion device for infusion of a liquid drug into a patient&#39;s body over an extended period of time and methods thereof are disclosed. The device includes a sensor assembly, which produces a sensor assembly output based on an infusion characteristic of the ambulatory infusion device and based on a supply voltage/current, and a supply unit which is coupled to a sensor of the sensor assembly and generates the supply voltage/current. A sensor testing unit detects a failure of the sensor assembly, wherein the sensor testing unit is coupled to the sensor assembly and the supply unit, and the sensor testing unit carries out a sensor testing sequence. The sensor testing sequence includes controlling the supply unit so as to produce a variation of the supply voltage/current, and determining whether the variation of the supply voltage/current produces a corresponding variation of the sensor assembly output.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is filed under 35 U.S.C. §111(a) as acontinuation of U.S. application Ser. No. 12/761,445, filed Apr. 16,2010, which claims the benefit of European Patent Application No.EP09005407.3, filed Apr. 16, 2009.

TECHNICAL FIELD

The present disclosure generally relates to an ambulatory infusiondevice for the infusion of a liquid drug into a patient's body over anextended time period. The infusion device comprises a sensor testingunit for detecting a failure of a sensor assembly of the infusiondevice. The present disclosure is also directed towards a correspondingmethod for detecting a failure of a sensor assembly.

BACKGROUND

As background, ambulatory infusion devices for infusion of a liquid drugover an extended time period are known in the art for a number oftherapies. In particular, such devices may form the basis for astate-of-the-art therapy for Diabetes Mellitus by CSII (ContinuousSubcutaneous Insulin Infusion). Such an ambulatory infusion device isdisclosed, for example, in PCT Patent Application Publication No.WO/2003/053498, to which reference is made for the general design andfeatures of such devices according to the state of the art. Anambulatory infusion device according to the technical field as statedabove and, more particularly, an ambulatory infusion device inaccordance with the present disclosure may be referred to simply as a“device” or an “infusion device.”

Besides diabetes therapy, ambulatory infusion devices may be used for anumber of other therapies, such as cancer treatment or pain therapy,without requiring substantial modification. Although this disclosuremainly refers to diabetes (i.e., CSII) therapy, it is contemplated thatthe embodiments shown and described herein may be used for other typesof therapies as well, without being limited to this specificapplication.

SUMMARY

In one embodiment, an ambulatory infusion device for infusion of aliquid drug into a patient's body over an extended period of timecomprises: a sensor assembly having a sensor, wherein the sensorassembly produces a sensor assembly output based on an infusioncharacteristic of the ambulatory infusion device and based on a supplyvoltage/current; a supply unit, wherein the supply unit is coupled tothe sensor and generates the supply voltage/current; and a sensortesting unit which detects a failure of the sensor assembly, wherein thesensor testing unit is coupled to the sensor assembly and the supplyunit, and the sensor testing unit carries out a sensor testing sequence,the sensor testing sequence comprising: controlling the supply unit soas to produce a variation of the supply voltage/current, and determiningwhether the variation of the supply voltage/current produces acorresponding variation of the sensor assembly output.

In another embodiment, a method for detecting a failure of a sensorassembly of an ambulatory infusion device, wherein the sensor assemblycomprises a sensor and produces a sensor assembly output based on aninfusion characteristic of the ambulatory infusion device and based on asupply voltage/current coupled to the sensor, comprises: producing avariation of the supply voltage/current, and determining whether thevariation of the supply voltage/current produces a correspondingvariation of the sensor assembly output.

These and additional features provided by the embodiments describedherein will be more fully understood in view of the following detaileddescription, in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the scope of the claims. Thefollowing detailed description of the illustrative embodiments can beunderstood when read in conjunction with the following drawings, wherelike structures are indicated with like reference characters and inwhich:

FIG. 1 depicts a block diagram of an ambulatory infusion deviceaccording to one or more embodiments shown and described herein;

FIGS. 2A-B depict a tester sensing sequence according to one or moreembodiments shown and described herein;

FIGS. 3A-C depict graphs of a supply voltage/current and a sensorassembly output according to one or more embodiments shown and describedherein;

FIG. 4 depicts a schematic of an ambulatory infusion device according toone or more embodiments shown and described herein;

FIG. 5 depicts a schematic of an ambulatory infusion device according toone or more embodiments shown and described herein; and

FIGS. 6A-B depict ambulatory infusion devices according to one or moreembodiments shown and described herein.

DETAILED DESCRIPTION

The various embodiments described herein generally relate to ambulatoryinfusion devices for infusion of a liquid drug into a patient's bodyover an extended time period. In particular, embodiments herein relateto detecting failure of a sensor (or sensors) used by the infusiondevice. Such sensors may be used to supervise overall operation of thedevice and to detect the occurrence of hazardous situations (e.g.,fluidic leakages and occlusions). Often devices are of thesyringe-driver type and comprise a force sensor as part of the drivesystem which measures the reaction force that is exerted by the drivesystem onto the plug of a drug cartridge when infusing the drugcontained in the drug cartridge. Alternatively or additionally, infusiondevices may comprise a fluidic pressure sensor or a drug flow sensor,for example, at the drug cartridge outlet or directly at the infusionsite. Since the primary purpose of such sensors may be the earlydetection of occlusions (i.e., blockages of the infusion line), they mayalso be referred to as “occlusion sensors.” Proper operation of suchocclusion sensors may be important for the overall system and patientsafety.

Typically, a sensor in an ambulatory infusion device may be disposed ata position which is remote from the other electronic components (e.g.,near the drug cartridge) and therefore may require cabling such as, forexample, a flexible printed circuit board, discrete wires, or othersuitable means. In some embodiments, the sensor may be a disposablesensor which is connected via releasable electrical connectors (e.g., asensor at the infusion site). Since cabling, wiring, and connectors havebeen known to be susceptible to failure (e.g., opens, shorts, damagedwires, loose contacts, intermittent contacts, etc.), it may be desirableto assure the integrity of the sensor output (and, hence, the cabling,wiring, and/or connections to the sensor) and to safely detect anyfailures.

As one example, U.S. Patent Application Publication No. 2007/0149926discloses a type of fault detection in a syringe-driver device whichcompares two independent force measurements. In particular, it isdetermined whether the level of the motor current (which may beindicative of the infusion force) corresponds to a force signal measuredby a force sensor. This approach may be limited in that it is linked tospecific hardware architectures and may be dependent on a considerablenumber of system parameters.

Embodiments of the present disclosure help to improve the operation ofambulatory infusion devices and to ensure reliable, cost-effectivedetection of failures for many different types of device architectures.Many sensors that are typically used in ambulatory infusion devices(e.g., occlusion sensors, including the subsequent measurement circuitryto which they are connected) produce outputs that are dependent on thesupply voltage/current to which the sensor is coupled (e.g., in order toprovide an operating voltage or current for the sensor). In accordancewith this disclosure, proper operation (or a fault) of the sensorassembly may be detected by varying the supply voltage/current anddetermining if this variation is properly reflected by a correspondingvariation of the sensor assembly output.

For purposes of this disclosure, the term “output” refers to a physicalcharacteristic, such as pressure, force, voltage, current, ohmicresistance, and the like. It also refers to the numeric representationof such a physical characteristic as obtained, for example, byanalog-to-digital conversion of such a characteristic. The value of anoutput may be constant or changing over time. For simplificationpurposes, a specific value of an output may also be referred to as“output” if the meaning is obvious within the context.

For purposes of this disclosure, the term “infusion characteristic”refers to a physical characteristic or property of the infusion devicewhich is measured by the sensor of the sensor assembly and which isassociated with the drug infusion or administration. The infusioncharacteristic may, for example, include a pressure of the drug or anyother physical characteristic which corresponds to the pressure of thedrug, such as the reaction force exerted by a drive system of thedevice.

For purposes of this disclosure, the term “sensor assembly” refers to anelectrical sensor for sensing a physical characteristic, in particularan infusion characteristic, and the associated measurement circuitrywhich is typically used for filtering, amplification, analog-to-digitalconversion, and the like. In one embodiment, the sensor may be anocclusion sensor as described above.

For purposes of this disclosure, the term “sensor assembly output”refers to the output which is generated as the overall output of thesensor assembly. The sensor assembly output may include an analog signal(e.g., voltage or current) or a number representing the digitized analogsignal output (e.g., digitized by an analog-to-digital converter). Theterm “sensor output” refers to an output which is generated or varied bythe sensor based on the infusion characteristic. Examples of sensoroutputs include the electrical current through a force-sensitiveresistor (FSR) or the differential voltage of a resistive strain-gaugebridge under mechanical load.

For purposes of this disclosure, the term “supply voltage/current”refers to the output which is generated by a power supply. For aconstant voltage power supply, the supply voltage/current refers to thevoltage output generated by the power supply. For example, a constantvoltage power supply may generate an output of about 3 Volts that may beused to supply power to the sensor and to other circuitry in theinfusion device. For a constant current power supply, the supplyvoltage/current refers to the current output generated by the powersupply. For example, a constant current power supply may generate anoutput of about 4 milliamps that may be used to supply power to thesensor and to other circuitry in the infusion device. For a variablevoltage power supply, the supply voltage/current refers to the variablevoltage output generated by the power supply. For example, an AC voltagepower supply may generate an output of about 12 Volts peak-to-peak atabout 60 Hertz that may be used to supply power to the sensor and toother circuitry in the infusion device.

For purposes of this disclosure, the terms “administration of the drug,”“administering the drug,” and similar syntactical variations refer tothe infusion of the drug into the body of the patient.

FIG. 1 shows a block diagram of an infusion device according to oneembodiment of the present disclosure. The infusion device comprises asensor 102, a supply unit 104, measurement circuitry 112, a sensortesting unit 114, and a controller unit 116. The sensor 102 and themeasurement circuitry 112, in combination, form the sensor assembly. Thecontroller unit 116 may control and monitor the operation of theinfusion device. The controller unit 116 may include state-of-the-artcomponents such as one or more microcontrollers, memory, clockcircuitry, interface circuitry, and the like. Other components of theinfusion device such as a drive unit, a drug cartridge, a userinterface, and the like are not shown in FIG. 1 but are well known for aperson skilled in the art. The overall design of the infusion devicemay, for example, be in accordance with the device as disclosed in PCTApplication Publication No. WO/2003/053498 or any commercially-availableinsulin infusion pump.

The characteristics of the sensor 102 may be assumed to be such that thesensor output is proportional to a supply voltage/current. It may alsobe assumed that the transfer characteristics of the measurementcircuitry 112 are linear, such that the sensor assembly output of thesensor assembly is expected to be proportional to the supplyvoltage/current as well. None of these assumptions, however, is requiredas long as the corresponding relationship is unique. For example, theremay be a non-linear relationship between the supply voltage/current andthe sensor output. However, the sensor testing unit 114 may be able todetermine whether the sensor is operating properly as long as therelationship between the supply voltage/current and the sensor output isknown.

The supply unit 104 is configured to supply the sensor 102 with thesupply voltage/current. As discussed herein, the supply voltage/currentmay be varied between two discrete levels such as two different supplyvoltage levels or two different supply current levels. The sensorassembly output may be pre-processed and sampled by the measurementcircuitry 112. The measurement circuitry 112 typically comprisespre-processing circuitry such as amplifiers and/or signal conditioningcircuitry, a sample-and-hold circuit, and an analog-to-digital converteras is known in the art. The measurement circuitry 112 may couple thesensor assembly output to the sensor testing unit 114 as well as to thecontroller unit 116. The general evaluation of the sensor assemblyoutput for supervising the administration of the drug and the detectionof sensor failures (and other faults) may be performed by the controllerunit 116.

For testing the sensor assembly, the sensor testing unit 114 may controlthe supply unit 104 so as to produce a variation of the supplyvoltage/current. The sensor testing unit 114 may then be capable ofdetecting whether or not the variation of the sensor assembly outputcorresponds to the variation of the supply voltage/current. If this isnot the case, the sensor testing unit 114 may send a corresponding faultsignal to the controller unit 116 which may initiate fault handlingsteps such as terminating the drug administration and/or alarming theuser via a user interface (e.g., by an optical, acoustical, or tactilesignal). Testing of the sensor assembly may be initiated by thecontroller unit 116 and may be carried out in fixed time intervals suchas, for example, every 1 minute or every 3 minutes. In addition oralternatively, the testing may be carried out before, during, and/orafter a drug administration.

FIGS. 2A and 2B depict a sensor testing sequence according to oneembodiment shown and described herein. Graph 120 of FIG. 2A illustratesthe supply voltage/current, S, and graph 130 of FIG. 2B illustrates thesensor assembly output, M, both as functions of time, t. As shown inthis embodiment, the supply unit 104 may provide two discrete levels ofthe supply voltage/current S, namely a testing supply level S_(T) and anoperating supply level S_(O). The operating supply level S_(O) may bethe nominal supply level for which the sensor 102 is designed, and thetesting supply level S_(T) may be half of the operating supply levelS_(O). It to be understood that the testing supply level S_(T) maycomprise other levels as well (e.g., ⅓ or ⅔ of S_(O)), including levelsthat are higher than the operating supply level S_(O). Although notshown in FIGS. 2A and 2B, the supply unit 104 may also provide a supplyvoltage/current level corresponding to the sensor 102 being inactive(e.g., zero voltage or current).

In an initial state, the supply unit 104 may provide a supplyvoltage/current which results in the sensor 102 being inactive. That is,the supply voltage/current may be zero (e.g., 0 Volts or 0 mA), whichmay conserve energy by powering down the sensor (and other circuitry aswell) when the infusion device is not administering the drug. At a firstpoint in time, t₁₁, the supply unit 104 may be controlled by thecontroller unit 116 to provide the supply voltage/current S at thetesting supply level S_(T), as indicated by line 122. The resultingtesting output value M_(T) is indicated by line 132 and may be sampled(e.g., digitized) by the measurement circuitry 112. At the second pointin time, t₁₂, the supply unit 104 may be controlled by the controllerunit 116 to provide the supply voltage/current S at the operating supplylevel S_(O) as indicated by line 124. The resulting operating outputvalue M_(O) as shown by line 134 may be sampled by the measurementcircuitry 112.

The two sampled values M_(T) and M_(O) of the sensor assembly output maybe evaluated by the sensor testing unit 114 which may determine if therelationship

$\begin{matrix}{\frac{M_{T}}{S_{T}} = {{\frac{M_{O}}{S_{O}}\mspace{14mu}{or}\mspace{14mu}\frac{M_{T}}{M_{O}}} = \frac{S_{T}}{S_{O}}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$is satisfied (i.e., within an expected range). Generally, Eq. 1indicates the expected relationship between the supply voltage/currentand the sensor assembly output when the sensor assembly output isproportional to the supply voltage/current. As can be seen from thegraph 130, the change of the level of the supply voltage/current S attime t₁₂ is reflected by a proportional change of the level of thesensor assembly output M, thus indicating proper operation of the sensor102 and the measurement circuitry 112. The sensor 102 may be deactivatedat time t₁₃ (e.g., to save energy).

After switching the power supply from the testing supply level S_(T) tothe operating supply level S_(O) at time t₁₂, a drug administration maybe performed in the time interval from t₁₂ to t₁₃ (i.e., with the sensor102 being operated at its nominal supply level). Depending on thespecific administration regime of the infusion device, the sensor 102may be operated continuously during the administration or may beoperated in a non-continuous sequence. Additionally or alternatively,the sensor testing sequence may be carried out repeatedly during drugadministration.

Continuing to refer to FIGS. 2A and 2B, the sensor testing sequencestarting at time t₂₁ and ending at time t₂₃ shows a situation in whichthe sensor 102 and/or the measurement circuitry 112 may not be operatingproperly. As indicated by lines 122′, 124′, the sensor 102 isconsecutively supplied with the testing supply level S_(T) at time t₂₁and the operating supply level S_(O) at time t₂₂, as described above.However, the sensor assembly output M_(I) does not reflect thisvariation of the supply voltage, but stays constant, as indicated byline 138. It is to be noted that, for some types of faults, the sensorassembly output M may show a slight variation such as, for example, dueto noise that does not correspond to the variation of the supplyvoltage/current.

The proportional relation between the supply voltage/current S and thesensor assembly output M according to Eq. 1 may be based on a number ofassumptions and idealizations. In particular, it may be assumed that theinfusion characteristic such as a mechanical load which is acting on thesensor 102 stays relatively constant during the sensor testing sequence.This is likely since the response time of the mechanical load may besubstantially greater than the testing time (e.g., t₁₂−t₁₁).Furthermore, it may be assumed that neither the sensor output nor thesensor assembly output M has an offset component, a substantial amountnoise, or drift component.

When taking noise effects into account, the sensor testing unit 114 maydetermine whether the ratio of the sensor assembly output at thedifferent levels of supply voltage/currents fall within an expectedrange. The limits of the expected range may take into consideration thepredicted amount of noise and may be selected to be relatively large inorder to avoid false alarms. The expected range may also be selected tobe small enough to insure that it is based on the variation of thesupply voltage/current S. Also more advanced methods for filtering noise(e.g., statistical hypothesis testing for a given ratio or calculatingthe difference of the sensor assembly outputs) may be employed as well.

The time delay between taking the samples of the sensor assembly outputshould be relatively short in order to minimize the superimposed effectof a variation of the infusion characteristic. For the example shown inFIG. 2A, the sampling of the sensor assembly output M may be performedshortly before t₁₂ and t₂₂ for the testing supply level S_(T) andshortly after t₁₂ and t₂₂ for the operating supply level S_(O). Thistime delay may be, for example, 1 millisecond.

Referring to FIGS. 1, 2A and 2B, if the infusion device determines(e.g., via the sensor testing unit 114) that a variation of the supplyvoltage/current is not reflected by a corresponding variation of thesensor assembly output, the presence of a fault in the sensor assemblymay be assumed. When a fault is detected, the infusion device may beconfigured to carry out fault handling steps such as, for example,stopping the drug administration and alerting the user by an optical,acoustical, and/or tactile alarm.

The infusion device may detect the presence of a fault after a singlesensor testing sequence. However, since unwanted disturbances may beinadvertently introduced into the sensor assembly output, the infusiondevice may be configured to assume the presence of a fault only if thefault persists for a consecutive number of testing sequences. Suchunwanted disturbances may be introduced into the sensor 102 and/or thesensor assembly output by electrical or mechanical means such as, forexample, electrical noise, electromagnetic interference, shock,vibration, and so forth. In one embodiment, the sensor testing unit 114may be configured to repeat the sensor testing sequence at least oneadditional time if the first sensor testing sequence indicates thepresence of a fault.

The expected and corresponding variation of the sensor assembly outputwhen the supply voltage/current is varied may be determined by thedesign of the sensor and its physical operation principle as well as thesubsequent measurement circuitry as discussed herein. The output of somesensors may vary linearly when the supply voltage/current is varied,while the output of other sensors may vary non-linearly when the supplyvoltage/current is varied. Thus, an expected range of the variation ofthe sensor assembly output may be established, if necessary.

In some embodiments, the sensor assembly comprises a sensor 102 with atleast one force-sensitive resistor, pressure-sensitive resistor, and/orstrain gauge. A force-sensitive resistor (FSR) may, for example, becoupled to the drive system of the infusion device according to thedisclosure of U.S. Pat. No. 6,485,465. Alternatively, a bending beam andstrain gauge arrangement may be used for the force measurement. In someembodiments, the sensor assembly comprises a strain-gauge measurementbridge for measuring the reaction force, for which a correspondingdesign is disclosed in PCT Patent Application No. WO/2001/24854. Thesensor 102 may be advantageously mechanically and/or electricallypre-loaded to always ensure a non-zero sensor output signal.

In some embodiments, the supply unit 104 may comprise a variable voltagesupply or a variable current supply, depending on the type of the sensorused. Accordingly, in one embodiment, the supply voltage/current may bea supply current if the sensor is designed for controlled current supplyor may be a supply voltage if the sensor is configured for controlledvoltage supply. Both kinds of sensors are known in the art and arecommercially available in a large variety.

As shown in FIGS. 2A and 2B, the supply unit 104 may be configured toprovide a set of at least two discrete levels of the supplyvoltage/current, such as different supply voltage levels or supplycurrent levels. This may be achieved by various means such as using atleast two power supplies. It may alternatively be achieved by acombination of one supply and additional resistors, such as droppingresistors or shunts, and a supply selector which is controlled by thesensor testing unit for switching between the levels in order to varythe supply voltage/current. The supply selector may comprise solid statecircuit such as a multiplexer.

As discussed herein, the operating supply level may be the standardsupply level for which the sensor is designed and may be normally usedfor measuring the infusion characteristic (e.g., when the infusiondevice is administering the drug to the patient). For this type ofembodiment, the sensor testing sequence may comprise switching thesupply voltage/current between the two discrete supply levels one ormore times. The corresponding graphical representation of the supplyvoltage/current may be a step or a square wave.

As an alternative to two supply levels, the supply unit 104 may beconfigured to provide three or more discrete supply levels, and thesensor testing sequence may comprise switching between these multiplesupply levels when varying the supply voltage/current. In anotherembodiment, the supply unit 104 may be configured to provide acontinuously varying supply voltage/current, for example, of thesinusoidal wave type or the triangular wave type. In this disclosure,two discrete supply levels are shown and described, although it is to beunderstood that more than two discrete supply levels may be used. Themodifications required for three or more discrete supply levels or for acontinuously varying supply voltage/current are known in the art.

In some embodiments, the expected variation of the sensor assemblyoutput when the supply voltage/current is varied may be proportional tothe variation of the supply voltage/current. Several types of sensorsgenerate an output which is directly or inversely proportional to thelevel of the supply voltage/current. This may be the case, for example,for the current through an FSR when the FSR is supplied with a fixedvoltage, or for the voltage drop across an FSR when the FSR is suppliedwith a fixed current. This may also be the case for the differentialvoltage across sensor bridge when the sensor bridge is supplied by afixed voltage or current. The sensor assembly output may also beproportional to the variation of the supply voltage/current for therectified output of capacitive sensors, inductive sensors, or sensorbridges when supplied by a sinusoidal AC supply.

If the sensor output is proportional to a supply voltage/current, thesensor assembly output may also be proportional to the supplyvoltage/current, assuming the measurement circuitry 112 is linear. Incase of an offset (e.g., voltage or current offset), the absolute valueof the sensor assembly output may not be directly proportional to theabsolute value of the supply voltage/current. The expected variation ofthe sensor assembly output, however, may be proportional to thevariation of the supply voltage/current if the offset is taken intoconsideration. If the sensor 102 exhibits substantially non-linearcharacteristic, the measurement circuitry 112 may compensate for this byapplying linearization techniques known in the art. As discussed herein,an expected proportional relationship may allow the sensor testing unitto determine if the variation of the supply voltage/current is reflectedin the expected variation of the sensor assembly output by simplemathematical (e.g., division) operations.

In some embodiments, the sensor testing unit 114 may be configured tomeasure the value of the supply voltage/current. Measuring the actualvalue of the supply voltage/current may be advantageous for moreaccurately determining the relationship between the variation of thesupply voltage/current and the sensor assembly output. This may be thecase if the supply voltage/current can vary, for example, if the powersupply is a battery and/or if the level of the supply voltage/currentcan be influenced by the sensor or other factors. An example of such apower supply may be a constant voltage power supply having anon-negligible internal series impedance which supplies power to an FSR.

In some embodiments, varying the supply voltage/current may be carriedout by switching a single power supply between at least two discretesupply voltage/current levels (e.g., at least two discrete voltage orcurrent levels). The supply voltage/current may, as discussed herein, bea supply voltage or a supply current. The switching may be performed byselecting one of at least two independent supplies or by varying asingle supply. Alternatively, the sensor testing sequence may comprisevarying the supply voltage/current continuously by providing, forexample, a sinusoidal wave or triangle wave voltage or current supply.

In some embodiments, the expected variation of the sensor assemblyoutput upon a variation of the supply voltage/current may beproportional to the variation of the supply voltage/current. For thoseembodiments, the sensor testing sequence may comprise determiningwhether the variation of the sensor assembly output is proportional tothe variation of the supply voltage/current. For example, the supplyunit may generate a supply voltage/current of 3 V, which produces asensor assembly output of about 1 V; the supply voltage/current may bevaried to be about 1.5 V, which produces a sensor assembly output ofabout 0.5 V. In this example, the sensor assembly output is proportionalto the supply voltage/current (i.e., 3V/1.5V=1V/0.5V).

In some embodiments, the sensor testing sequence comprises varying thesupply voltage/current such that the expected variation of the sensorassembly output is relatively large as compared to a variation of thesensor assembly output resulting from a variation of the infusioncharacteristic while carrying out the sensor testing sequence. Thisimplies that the variation of the supply voltage/current should be largeand that the steps of varying the supply voltage/current and samplingthe sensor assembly output should be carried out in a time intervalwhich is short as compared to the expected variation of the measurementsignal. For noise reducing purposes, the sensor testing may furthercomprise repeatedly varying the supply voltage/current and sampling thesensor assembly output as will be described below in more detail.

For improved immunity to noise, it may be advantageous to expect thevariation of the output signal to be within an expected range ratherthan having a precisely defined value. The limits of such an expectedrange may depend on the expected level of noise and/or distortion. Forsuch an embodiment, the variation of the output signal corresponds tothe variation of the supply voltage/current if it is within the expectedrange.

In some embodiments, sensor testing may be carried out in defined timeintervals prior to, during, and/or after drug administration by theinfusion device. The defined time intervals may be short enough todetect a fault of the sensor assembly without significant delay, forexample, every second, every minute, or every three minutes. For deviceswhich do not administer the drug continuously but are configured toadminister drug boli on demand and/or to administer drug continuously ina pulsed fashion (e.g., with a drug pulse being administered with timeintervals of several minutes), the sensor testing may be carried outprior to and/or after the drug administration. If larger drug amounts,such as drug boli, are administered by administering separated drugpulses, a sensor testing sequence may additionally or alternatively becarried out between the administrations of the single drug pulses.

FIGS. 3A-C depict graphs of the supply voltage/current and a sensorassembly output according to the system of FIG. 1 in which the immunityto noise is increased by averaging multiple samples. Graph 220 of FIG.3A illustrates the supply voltage/current S as a function of time t. Thesupply unit 104 is controlled so as to repeatedly supply the sensor 102with the testing supply level S_(T) followed by the operating supplylevel S_(O). This results in an overall square wave signal with thetimes t₁, t₂, . . . , t₈ indicating the middle of each segment. As shownin FIG. 3B, the sensor assembly output M may be sampled at the times t₁,t₂, . . . , t₈, as shown by sampling arrows 262 a, 262 b, . . . , 262 h.The values sampled at the times t₁, t₃, t₅, t₇ are testing output valuesM_(T) while the values sampled at the times t₂, t₄, t₆, t₈ are operatingoutput values M_(O). Arrows in FIG. 3B indicate a sample of M with thelength of the arrow corresponding to the amplitude of the sampled value.It can be seen that the sampled values of the sensor assembly output Mgenerally reflect the variation of the supply voltage/current S.However, the value of the sensor assembly output M may not be preciselyconstant at either of the supply voltage/current levels, but may showsome additional variation due to noise.

The values of the sensor assembly output M sampled in adjacent segmentsof the supply voltage/current S may be evaluated pair wise. Therefore,the sensor testing unit 114 may compute an average of M_(T)/M_(O) asfollows:

${\frac{M_{T}}{M_{O}}({avg})} = {\frac{1}{4}{\left( {\frac{M_{T{(t_{1})}}}{M_{O{(t_{2})}}} + \frac{M_{T{(t_{3})}}}{M_{O{(t_{4})}}} + \frac{M_{T{(t_{5})}}}{M_{O{(t_{6})}}} + \frac{M_{T{(t_{7})}}}{M_{O{(t_{8})}}}} \right).}}$Because many noise effects and in particular variations of the infusioncharacteristic may occur slowly in comparison with switching the levelof the supply voltage/current S and because taking adjacent samples ofthe sensor assembly output M, the ratio M_(T)/M_(O) may largely beindependent of those effects as shown by the dots 282 in FIG. 3C. Thedots indicate the computed quotients M_(T)/M_(O) according to theformula given above.

In another embodiment, the averaging may be performed such that a ratiois calculated for each M_(T) using the adjacent M_(O) on either side,and likewise a ratio is calculated for each M_(O) using the adjacentM_(T) on either side. Duplicate ratios are discarded. Therefore, thesensor testing unit 114 may compute an average of M_(T)/M_(O) asfollows:

${\frac{M_{T}}{M_{O}}({avg})} = {\frac{1}{7}\left( {\frac{M_{T{(t_{1})}}}{M_{O{(t_{2})}}} + \frac{M_{T{(t_{3})}}}{M_{O{(t_{2})}}} + \frac{M_{T{(t_{3})}}}{M_{O{(t_{4})}}} + \frac{M_{T{(t_{5})}}}{M_{O{(t_{4})}}} + \frac{M_{T{(t_{5})}}}{M_{O{(t_{6})}}} + \frac{M_{T{(t_{7})}}}{M_{O{(t_{6})}}} + \frac{M_{T{(t_{7})}}}{M_{O{(t_{8})}}}} \right)}$are computed. That is, except the first and last samples, each sampledvalue of the sensor assembly output M is used for the computation of twoM_(T)/M_(O) ratios. More advanced filtering and noise reductiontechniques may be used as well, but may not be required.

As further illustrated in FIGS. 3A-B, a series of five consecutive drugpulses may be administered by the infusion device after completing thesensor testing sequence (e.g., after t₈). For the administration of eachof the drug pulses, the sensor 102 may be temporarily supplied with theoperating supply level S_(O) as indicated by the pulses 252. During orafter completing the administration of each drug pulse, the infusioncharacteristic may be determined by sampling the sensor assembly outputM, as indicated by the sampling arrows 272 a, 272 b, . . . , 272 e,followed by an evaluation in the controller unit 116. The shownvariation of the sampled measurement currents may be typical of theinfusion characteristic, particular of the force or pressure which isacting on the sensor 102 during administration of the drug.

In the embodiments of FIGS. 2A, 2B, and 3A, the testing supply levelS_(T) is depicted as about half of the operating supply level S_(O).Accordingly, the expected testing output value M_(T) may also be half ofthe operating output value M_(O). It will the appreciated by a personskilled in the art that other ratios of the testing supply level S_(T)and the operating supply level S_(O) may be used as well, and that thetesting supply level S_(T) may also be higher than the operating supplylevel S_(O). It will further be appreciated that the device and thetesting method may be modified in a straightforward way so as to havemore than two levels of the supply signal or have a continuous variationof the supply signal (e.g., a sine wave or a triangular wave).

FIG. 4 depicts a more detailed view of a hardware structure for thesystem of FIG. 1. The sensor 102 may comprise an FSR which measures thereaction force exerted on the drug in the drug container (not shown) asdisclosed, for example, in U.S. Pat. No. 6,485,465. Alternatively, thesensor 102 may be a pressure sensitive resistor which measures thefluidic pressure of the drug at the outlet of the drug cartridge or maybe a strain gauge which is mounted onto a bending beam. In thisembodiment, the supply voltage U which is supplied to the sensor 102 isthe supply voltage/current S and the resulting current I through thesensor 102 is the sensor output. The measurement circuitry 112 maycomprise a current-to-voltage converter followed by an analog-to-digitalconverter (ADC), such that the output signal M is a number which isproportional the current I through the sensor 102 when the sensorassembly operates properly. For a constant value of the infusioncharacteristic, that is, the mechanical load which is acting on thesensor 102, and thus a given ohmic resistance of the sensor 102, thecurrent I may be proportional to the supply voltage. The resistance ofthe sensor 102 may be either directly or inversely proportional to theacting force or pressure or may have any defined non-linearcharacteristics.

In the embodiment shown in FIG. 4, the supply unit 104 may comprise atesting voltage supply 108 for providing a testing supply voltage U_(T)and an operating voltage supply 110 for providing an operating supplyvoltage U_(O). These voltages correspond to the testing supply levelS_(T) and the operating supply level S_(O) as described above. Thesupply unit 104 may further comprise a supply selector 106 foralternatively connecting the sensor 102 to either of the testing voltagesupply 108, the operating voltage supply 110, or ground U_(G). Thevoltage supplies 108, 110 may be two different voltage supplies as shownin FIG. 4 or may alternatively be implemented as a single variablevoltage supply. In either case, the sensor testing unit 114 may control(e.g., via the supply selector 106 or other suitable means) the voltagesupply to the sensor 102 to be either the testing supply voltage U_(T),the operating supply voltage U_(O), or ground U_(G). If more than twolevels of testing supply voltage are used, the sensor testing unit 114may control the voltage supply to the sensor 102 to be any one of thelevels of the testing supply voltage.

The testing supply voltage U_(T) and the operating supply voltage U_(O)may be selected in accordance with the sensor specification such thatthe sensor output is always valid (e.g., readable) by the measurementcircuitry 112. In a typical embodiment, the measurement circuitry 112comprises an operational amplifier and an ADC with a unipolar powersupply (i.e., a positive voltage supply with respect to ground), asdescribed below in more detail with reference to FIG. 5. In this case,the amplifier and the ADC may only be capable of processing positivevoltage inputs within a certain range (and not inputs which arenegative). For this type of embodiment, a testing supply voltage U_(T)which is in a range of ⅓ to ⅔ of the operating supply voltage U_(O) maybe used. However, if the measurement circuitry 112 is supplied with abipolar supply voltage with respect to ground, the ADC may be capable ofprocessing both positive and negative inputs.

Instead of the constant voltage supplies 108, 110 shown in FIG. 4,constant current supplies may be used to alternatively supply the sensor102 with a testing supply current and an operating supply current. Inthis case, the voltage drop across the sensor 102 may be used as sensoroutput and the measurement circuitry 112 may be configured to sample andmeasure this voltage drop. For selecting the operating supply level andthe testing supply level, the same considerations as given for a voltagesupply hold true. Furthermore, a variable voltage or current supply maybe used in which the voltage or current, respectively, varies with time(e.g., a sinusoidal or triangular wave output).

FIG. 5 schematically shows the hardware structure of another exemplarydevice in accordance with this disclosure. With respect to theoperation, reference is additionally made to FIGS. 2A, 2B, and 3A aswell as the corresponding description as given above. The sensor of thisembodiment may be realized as resistive strain-gauge bridge 310 whichcomprises resistors 310 a, 310 b, 310 c, 310 d having resistancesR_(310a), R_(310b), R_(310c), R_(310d), respectively The resistors maybe assembled on a bending beam carrier by adhesive bonding, thick-filmtechnologies, or other suitable method. The strain-gauge bridge 310 maybe supplied via a pair of opposed supply connectors 311 a, 311 b with asupply voltage U_(S) (e.g., supply connector 311 a is connected toU_(S), and supply connector 311 b is connected to ground). Thedifferential voltage U_(D), which may be measured between the other pairof opposed connectors 312 a, 312 b, may be the sensor output. Theoverall bridge resistance R_(B) between the supply connectors 311 a, 311b is typically in a range of 5 kilohms to 100 kilohms.

For this embodiment, the ambulatory infusion device may be of thesyringe-driver type, and resistors 310 a, 310 b, 310 d, 310 d may bepart of a strain-gauge beam arrangement, which measures the reactionforce which is exerted by a drive system onto the plug of a drugcartridge in the infusion device. An example of a correspondingmechanical structure is disclosed in PCT Patent Application PublicationNo. WO/2001/024854. Alternatively, the strain-gauge bridge 310 may bepart of a fluidic drug pressure sensor. Advantageously, the strain-gaugebridge 310 may be mechanically pre-loaded. The device may furthercomprise a microcontroller 305 which may integrate major components ofthe controller unit of the infusion device, the sensor testing unit, aswell as part of the supply unit and the measurement circuitry. This maybecome more readily apparent in the following description.

The resistors 310 a, 310 b, 310 d, 310 d may be arranged such that thebridge resistance R_(B) which can be measured between the supplyconnectors 311 a, 311 b stays substantially constant while theresistances R_(310a), R_(310b), R_(310c), R_(310d) depend on themechanical load that is acting on the sensor. For this arrangement, theratio U_(D)/U_(S) (i.e., the differential voltage U_(D) of the sensoroutput divided by the supply voltage U_(S) as the supplyvoltage/current) is proportional to mechanical load that is acting onthe bridge, independent of the absolute value of the supply voltageU_(S). The differential voltage U_(D) may be amplified by an amplifiercircuit 325 which comprise an instrumentation amplifier or one or moreoperational amplifiers. The output of the amplifier circuit 325generates an output voltage U_(M) (i.e., with respect to ground). Theoutput voltage U_(M) may be fed into an analog input IN₁ of themicrocontroller 305 and may be sampled and digitized by an ADC, forexample a 12-bit ADC which may be integrated with the microcontroller305. The output of the ADC may form the sensor assembly output M (e.g.,within the microcontroller 305).

The microcontroller 305 may comprise two voltage output ports OUT₁,OUT₂, to which the strain-gauge bridge 310 may alternatively supplied.If either of the outputs OUT₁, OUT₂ is active and provides an outputvoltage, the other of the two outputs may reconfigured to be in aninactive state, a high-impedance state, or an input port. The two outputports OUT₁, OUT₂ may provide the same standard supply voltage U₀ whichis given by or derived from the power supply voltage of the infusiondevice (and the microcontroller 305) and may, for example, be about 3 Vfor typical state-of-the-art technology. If the output port OUT₁ isactive, the standard supply voltage U₀ is directly connected to thestrain-gauge bridge 310 as operating supply voltage U_(O). If the outputport OUT₂ is active, the standard supply voltage U₀ is connected via anadditional dropping resistor 315 of constant resistance R_(D) as testingsupply voltage U_(T). The resistance R_(D) of the dropping resistor 315is typically chosen to have a similar value as the bridge resistanceR_(B).

If the strain-gauge bridge 310 is supplied via the dropping resistor315, the dropping resistor 315 and the strain-gauge bridge 310, incombination, form a voltage divider for the output voltage U₀ asprovided by the microcontroller 305. With R_(B) being the bridgeresistance, the testing supply voltage U_(T) is accordingly given by:

$U_{T} = {{U_{O} \times r\mspace{14mu}{and}\mspace{14mu} r} = \frac{R_{B}}{R_{B} + R_{D}}}$with r being a reduction factor. The voltage divider may besubstantially symmetrical if the condition R_(D)≈R_(B) is fulfilled.When the sensor assembly operates properly, the differential voltageU_(D), the output voltage U_(M) and, thus, the sensor assembly outputs Mmay be reduced by the same reduction factor r as the supply voltageU_(S) when the supply is switched from output OUT₁ to output OUT₂. Thismay correspond to the expected proportional relationship given by Eq. 1above. In this embodiment, the supply voltage/current is a supplyvoltage provided by the microcontroller 305 (i.e., via OUT₁ and OUT₂).The operating supply level S_(O) is given by the standard supply voltageU₀ and the testing supply level S_(T) is given by r×U₀ due to thevoltage drop introduced by the dropping resistor 315.

In practice, the nominal resistances of the resistors 310 a, 310 b, 310c, 310 d, that is, the resistances in the unloaded state of thestrain-gauge bridge 310, may have a considerable manufacturingtolerance. In addition, the standard supply voltage U₀ may not beconstant over time. Therefore, the supply voltage U_(S) may additionallybe measured via a second analog input IN₂ of the microcontroller 305 andthis measured voltage may be used for the computation. This may reduceor eliminate any noise or uncertainty introduced by the supplyvoltage/current.

Most of the components of FIG. 5 may be typically arranged on one ormultiple printed circuit boards of the device. The strain-gauge bridge310 may be connected with the other components via wiring elements suchas Flexible Printed Circuit Board (Flexprint), a Flexible CableConnector (FCC) or via single cables, as indicated by the dashed boxes320. Those wiring elements may be assembled in a bent condition and,therefore, in stress-loaded state according to the overall design andthe arrangements of the components of the infusion device.Alternatively, the amplifier circuit 325 may be disposed at thestrain-gauge bridge 310. In this case, the output U_(M) of the amplifiercircuit 325 may be connected with the microcontroller 305 via wiringcomponents. Wiring components as well as soldered connections are knownto be particularly susceptible to loose contacts as well as opens andshorts. In a design according to FIG. 5, the occurrence of such afailure results in the input of the amplifier circuit 325 floating(i.e., not being actively driven to a known state) and the measurementvoltage U_(M) as sensor assembly output of the amplifier circuit 325therefore not properly reflecting a corresponding variation of thesupply voltage U as expected. Similarly, the sensor assembly output willnot properly reflect the variation of the supply voltage/current if anyother component of the sensor assembly, in particular the amplifiercircuit 325, the strain-gauge bridge 310 or the ADCs of themicrocontroller 305 has an electrical failure.

It can be seen that the embodiment of the disclosure as shown in FIG. 5requires only one additional hardware component, namely the droppingresistor 315. Implementation of the method for verifying correctoperation of the sensor assembly is performed by some basiccomputational steps which are implemented in the microcontroller 305(e.g., computer instructions executed by the microcontroller 305).Accordingly, proper operation (or a failure) of the sensor assembly maybe detected with little additional hardware and software effort, thuskeeping the cost to a minimum.

FIGS. 6A and 6B depict examples of ambulatory infusion devices 400, 410which have a sensor 402, 412 configured to measure an infusioncharacteristic thereof. In FIG. 6A, the infusion device 400 comprises asensor 402, a drug cartridge 404 having a liquid drug 408, and a drivemechanism 406. When administrating the liquid drug 408, the drivemechanism 406 may produce a force which pushes the liquid drug 408 outof the drug cartridge 404 and into the body of the patient (not shown).In this embodiment, the sensor 402 may be coupled to the drive mechanism406 so as to measure a force exerted by the drive mechanism 406 whenadministering the drug. The use of the sensor 402 in this fashion may,as explained herein, permits the infusion device 400 to determinewhether an occlusion has occurred. Thus, the sensor 402 may be capableof measuring a force, which corresponds to an infusion characteristic ofthe infusion device 400.

Similarly, in FIG. 6B, the infusion device 410 comprises a sensor 412, adrug cartridge 414 having a liquid drug 418, and a drive mechanism 416.When administrating the liquid drug 418, the drive mechanism 416 mayproduce a force which pushes the drug out of the drug cartridge 414 andinto the body of the patient (not shown). In this embodiment, the sensor412 may be coupled to the drug cartridge 414 so as to measure a pressureof the drug when the infusion device administers the drug. The use ofthe sensor 412 in this fashion, as explained herein, may permit theinfusion device 410 to determine whether an occlusion has occurred.Thus, the sensor 412 may be capable of measuring a pressure, whichcorresponds to an infusion characteristic of the infusion device 410.These are just two examples of infusions devices which may use thesensor testing methods and systems described herein to determine whetherone or more sensors are operating properly. It is contemplated that theembodiments described herein may be used for other types of infusiondevices as well.

According to another aspect of this disclosure, the goal of improvingthe operation of ambulatory infusion devices may be achieved byproviding a method for detecting a failure of a sensor assembly of theambulatory infusion device by carrying out a sensor testing sequence.The sensor assembly may be configured to provide a sensor assemblyoutput based on an infusion characteristic and a supply voltage/currentprovided to a sensor of the sensor assembly. The sensor testing sequencemay comprise the steps of: producing a variation of the supplyvoltage/current, and determining whether the variation of supplyvoltage/current produces a corresponding variation of the sensorassembly output. In one embodiment, the method further may comprise thestep of sampling the sensor assembly output at different levels of thesupply voltage/current such as, for example, two discrete levels of thesupply voltage/current.

While particular embodiments and aspects have been illustrated anddescribed herein, various other changes and modifications may be madewithout departing from the spirit and scope of the invention. Moreover,although various inventive aspects have been described herein, suchaspects need not be utilized in combination. It is therefore intendedthat the appended claims cover all such changes and modifications thatare within the scope of this invention.

What is claimed is:
 1. An infusion device which is operable with asupply voltage/current and a measurement signal, said device comprising:a sensor which receives the supply voltage/current; and a measurementcircuit which receives the measurement signal from the sensor anddetermines whether a change in the supply voltage/current causes acorresponding change in the measurement signal, wherein the sensorprovides the measurement signal as a function of the received supplyvoltage/current and a measured characteristic of the infusion device. 2.The infusion device of claim 1, wherein the measured characteristic is apressure of a liquid drug or a characteristic corresponding to thepressure of the liquid drug.
 3. The infusion device of claim 1, furthercomprising a supply unit coupled to the sensor, wherein the supply unitgenerates the supply voltage/current.
 4. The infusion device of claim 3,wherein the supply unit comprises a variable voltage supply or avariable current supply.
 5. The infusion device of claim 3, wherein thesupply unit generates a continuously variable supply voltage/current. 6.The infusion device of claim 3, wherein the supply unit produces avariation of the supply voltage/current.
 7. The infusion device of claim6, wherein the variation of the supply voltage/current comprises two ormore discrete levels of the supply voltage/current.
 8. The infusiondevice of claim 6, wherein the variation of the supply voltage/currentcomprises two discrete levels of the supply voltage/current, a firstdiscrete level and a second discrete level.
 9. The infusion device ofclaim 8, wherein the first discrete level is about two times higher thanthe second discrete level.
 10. The infusion device of claim 8, whereinthe supply unit comprises at least one dropping resistor used forproducing the second discrete level.
 11. The infusion device of claim 1,wherein determining whether a change in the supply voltage/currentcauses a corresponding change in the measurement signal comprisesdetermining whether the corresponding change in the measurement signalis proportional to the change in the supply voltage/current.
 12. Theinfusion device of claim 1, wherein determining whether a change in thesupply voltage/current causes a corresponding change in the measurementsignal comprises determining whether the corresponding change in themeasurement signal falls within an expected range.
 13. The infusiondevice of claim 1, wherein determining whether a change in the supplyvoltage/current causes a corresponding change in the measurement signalcomprises averaging two or more samples of the sensor assembly output.14. The infusion device of claim 1, wherein the sensor comprises atleast one force-sensitive resistor, pressure-sensitive resistor, orstrain gauge.